This invention is directed to a quadrupole mass spectrometer and in particular to a quadrupole mass spectrometer having accelerating and decelerating grids at its output.
Mass spectrometers are in widespread use for gas analysis, particularly in combination with gas chromatographs for analysis of complex mixtures in the fields of organic chemistry, biochemical and biomedical analysis. Existing mass spectrometers particularly of the quadrupole mass spectrometer type have been dominant in relation to the above problems for the past few years because of some inherent advantages in design, particularly in relation to automated systems.
Present limitations to performance (sensitivity and resolution) of quadrupole mass spectrometers are set by the high mechanical precision required in the four-rod structure and by a limit to resolution due to the number of rf cycles the ions must spend in passing through the device. This depends on the ion energy but the latter cannot be lowered beyond a certain limit owing to deleterious effects of the dc fringing fields at the entrance, especially for high mass ions. Cost limitations are set by the complex precision circuitry required, especially for the exact control (&lt;1 part in 10.sup.5) of the rf/dc voltage ratio. A modification in the operating method for the mass filter was reported by U. Brinkmann in the International Journal of Mass Spectrometry Ion Physics 9 (1972) 161 and has been under investigation as reported by A. E. Holme et al in the publication International Journal of Mass Spectrometry and Ion Physics, 26(1978) pp 191-204. These devices operate with only an rf applied to the rod so that the electronic circuitry is much simpler. The mass separation depends upon the fact that ions whose trajectories are marginally stable with the particular applied rf voltage emerge with excess kinetic energies. This is probably partly due to an interplay between ion trajectories in the device and fringing fields at the ion exit. The length limitations to resolution no longer apply. The device geometry (mechanical tolerances) will be much less critical. The acceptance of ions should be much larger and fringing fields less important. Though promising results have been reported, there are two critical disadvantages. There can be a background signal due to higher mass ions or even high velocity ions which pass directly through the device near the central axis and are measured, and the system cannot include an electron multiplier detector with its important advantage of higher signal output levels and faster useful scan rates.